1. Field of the Invention
The field of the invention is that of communication networks. More precisely, the invention concerns 3rd generation mobile networks (also called 3G networks) such as in particular, although not exclusively, GSM/GPRS (Global System for Mobile communications/General Packet Radio Service), UMTS WCDMA (Universal Mobile Telecommunications Service Wideband Code-Division Multiple Access), UMTS TS-CDMA (Universal Mobile Telecommunications Service Time Slot Code-Division Multiple Access), etc. networks.
Even more precisely, the invention concerns a “load management” technique for facilitating the switching of a plurality of user equipments (3G mobile telephones, personal digital assistants (PDA), etc.) between a plurality of service states implemented in such networks.
The invention finds applications in particular in the management of overloading of a cell.
2. Description of the Prior Art
The 3GPP (3rd Generation Partnership Project) standard defines a plurality of service states of a user equipment.
As shown in FIG. 1, a user equipment (UE) can switch between four states:                a first or IDLE state in which the user equipment is not allocated any context and/or radio resources in the radio access network;        a second or Cell_DCH state in which the user equipment is allocated dedicated radio resources (a Dedicated Channel (DCH)) for real time transfer of large quantities of data in the context of a circuit call or a packet call;        a third or Cell_FACH state in which the user equipment is allocated common radio resources (a Forward Access Channel (FACH)), in other words in which the user equipment is not allocated its own radio resources, for transferring small quantities of data with no real time constraints in the context of a packet call only; and        a fourth or UR_PCH/CELL_PCH state in which the user equipment is not allocated any radio resource but retains a context, for example an international mobile subscriber identity (IMSI), in the radio network controller (RNC); the URA_PCH (UTRAN Registration Area Paging Channel) saves power (by releasing resources) and localizes the user equipment to the level of the nearest cell “URA” group.        
As a general rule, a user equipment enters the Cell_FACH state in three situations:                a first situation corresponding to the end of a period of activity: the user equipment has little data or no more data to transmit;        a second situation corresponding to a lack of radio resources in the Cell_DCH state: the user equipment has to wait in the Cell_FACH state until radio resources can be dedicated to it; and        a third situation corresponding to cell reselection: the user equipment in a given cell remains in the Cell_FACH state when it enters another cell.        
The drawbacks of the prior art are discussed hereinafter in the particular situation of access to a packet switched (PS) service by a user equipment.
In the context of access to packet services, if a user equipment has no or little data to transmit, it switches from the Cell_DCH state to the Cell_FACH state (arrow 1 in FIG. 1). In the Cell_FACH state the user equipment receives and transmits data on a secondary common control physical channel (SCCPCH), for which no radio resource is reserved per user equipment.
Reciprocally, if the user equipment has new data to transmit, it switches from the Cell_FACH state to the Cell_DCH state (arrow 2 in FIG. 1).
It is important to note that the user equipment changing to the Cell_DCH state is conditioned by the verification of an admission control algorithm, for example of the RAC (Radio Admission Control) type, consisting in determining if sufficient radio resources (in terms of power) are available for the user equipment.
Most existing RAC algorithms are “strict” in the sense that they allow only a very small number of user equipments of the same cell to be in the Cell_DCH state. For example, for a packet service at 64 kbps on the uplink channel and 384 kbps on the downlink channel, only three to seven user equipments per cell in the Cell_DCH state are allowed.
The inventors have found that these admission algorithms can be unsuitable in certain situations.
In fact, they may lead to overloading of the Cell_FACH state in a given cell because, for a “go transition” (1), a large number of user equipments is allowed to change from the Cell_DCH state to the Cell_FACH state (typically in the situation of consulting web pages, in which the user equipments use packet services with a non-continuous traffic model), whereas for a “return transition” (2) only a limited number of user equipments is allowed to return from the Cell_FACH state to the Cell_DCH state. Thus the drawback of these algorithms is that they cause the user equipments to remain longer in the Cell_FACH state with a very low bit rate and a very low quality of service (QoS), reflected in longer loading times for new web pages, for example.
At present the 3GPP standard does not define any “load management” or quality of service management technique for the Cell_FACH state.
However, a well known technique for facilitating the passage (2) of a user equipment from the Cell_FACH state to the Cell_DCH state is based on limiting the number of user equipments in the Cell_FACH state. The advantage of this solution is that it avoids overloading the Cell_FACH state.
However, this known technique is not adapted to the situation of overloading the Cell_DCH state. In fact, if a plurality of user equipments in the same cell became very active in the packet service (PS) packet domain (i.e. in the case of downloading and/or consulting web pages (web browsing)), then a limited number of those user equipments would change to the Cell_FACH state and a large number of them would remain locked in the Cell_DCH state, in which they would consume dedicated radio resources unnecessarily and would prevent user equipments in the IDLE state changing to the Cell_DCH state.
Another major drawback of this solution is that it cannot be adapted dynamically to the traffic situation and is therefore not the optimum for 3G networks given that those networks generally have variable traffic models.
There is therefore a requirement for optimization of load control in a cell of a 3G network, in particular to facilitate the switching of one or more user equipments between the various service states cited above.